Lighting for homes, offices, commercial spaces, and public areas may be controlled to account for occupancy and ambient light at the light fixture, workstation, room, floor and building levels. Some systems have been implemented using the Digital Addressable Lighting Interface (DALI) which is a global standard for a lighting control data protocol and transport mechanism maintained as IEC 62386. The DALI standard specifies a two wire, bi-directional data bus connecting a DALI application controller with up to 64 DALI controlled devices, referred to as control gear, such as ballasts, occupancy sensors, photo sensors, wall switches, and dimmers. The data bus cable is mains rated and may be run next to mains conductors or in a cable with mains conductors. The DALI control gear are individually addressable and data is transferred between the application controller and the control gear using an asynchronous, half-duplex, serial protocol. Data is transmitted using Manchester encoding at a fixed data transfer rate of 1200 bits/s to ensure reliable communications. The DALI bi-directional data bus also provides power at 16 volts and 250 mA maximum current. DALI application controllers and control gear may be connected in a star or daisy chain configuration.
FIG. 1 shows a block diagram of an exemplary DALI system 100. An application controller 105 is connected to a number of control gear 1100-11063 by the bi-directional data bus 115. Control gear 1100-11063 may control light sources 125 or other equipment or may be implemented as occupancy sensors, light sensors, wall switches or other lighting appliances. Mains power is provided through mains cable 120. In some implementations, mains power is provided by or controlled by application controller 105.
FIG. 2 shows a schematic diagram of at least a portion of an exemplary DALI control gear 205 similar to control gear 1100-11063. DALI control gear 205 may include a bus interface 210 and operating circuitry 215. Bus interface 210 may isolate the operating circuitry 215 from the bi-directional data bus 115 using a diode bridge 240 and optocouplers. For example, optocoupler 220R may be used for receiving commands or messages from application controller 105 to the control gear 205, while optocoupler 220T may be used for transmitting responses and messages from the control gear 205 to the application controller 105. The control gear 205 may include a computer 225, for example, a single chip microcontroller with a processor and memory 230 for exchanging information over the DALI bi-directional data bus 115 and for controlling lamps and other lighting equipment.
However, with this type of architecture, where one or more signals of the control gear are effectively coupled directly to the communications bus, some circuitry failures in the control gear may be capable of disabling the communications bus. In some failure modes of the control gear 205, one or more inputs or outputs of the microcontroller 225 may be pulled to a low or ground state and may remain at that state until the failure mode is resolved. For example, the microcontroller 225 may fail, resulting in a transmit output 235 being forced to a low or ground state. In the exemplary control gear 205 shown in FIG. 2, this causes a static voltage to be applied across the LED of optocoupler 220T which in turn causes the driver side of the optocoupler 220T to remain in an “on” or conductive state. This effectively shorts the two wire bi-directional data bus 115 through the diode bridge 240. As a result, no messages or responses may be conducted between the application controller 105 and the control gear 205 or any other devices that may be connected to the bi-directional data bus 115. It would be advantageous to provide a mechanism to avoid these conditions.